It is well-known that a provider/operator of a communications network strives to increase the level of "survivability" of the network whenever a fault occurs, e.g., a loss of signal is detected over a communications path. The operator typically increases the level of survivability by employing a protection-switching architecture to recover from a loss of signal condition. Protection switching, as it is commonly referred to, involves establishing preassigned backup resources, e.g., a duplicated/standby communications path, which may be switched into service when a loss of signal occurs.
The detection of a loss of signal is usually done simply by comparing the level of an optical signal received via a communications path with a predetermined threshold. If the level of the received optical signal is continuously below the threshold for a predetermined period of time, then protection switching may be triggered for that communications path.
It appears, however, that with the increasing deployment of Wavelength Division Multiplexing (WDM) and optical amplifiers in an optical network, the detection of a Loss of Signal (LOS) becomes more complicated. The reason for this is that an optical amplifier in a path which is experiencing an LOS may not immediately reduce its output power. As a result, the level of the received signal may not significantly change until each optical amplifier reduces its output power. Disadvantageously, user data may be lost, since a substantial amount of time, e.g., 50 milliseconds, may be exhausted before the LOS is detected.
The detection of a LOS is further complicated if the optical signal is composed of a plurality of optical signals of different wavelengths .lambda..sub.i.